This invention relates to a device for performing focus search smoothly in an optical type disc playback device adapted for playing back discs such as a video disc and a digital audio disc.
For optically picking up information recorded on a disc in an optical type disc playback device, an object lens of an optical pickup head is displaced in a vertical direction to conform to variations in the distance between the object lens and the surface of the disc so that a laser beam for detecting the recorded information will accurately focus on the disc surface. This control operation will hereinafter be referred to as "focus servo".
Since this focus servo has a limited operation range in which the focus servo control can be made, if the object lens is out of this operation range, the focus servo is temporarily suspended and the object lens is pulled back to a reference position, for example, the lowermost position. Then, the object lens is compulsorily moved slowly to enter the operation range. This operation will hereinafter be referred to as "focus search". FIG. 1 shows an example of a prior art focus search circuit.
In FIG. 1, laser beam 11 is irradiated on an optical disc 16 from a laser beam source 13 in an optical pickup head 15 through an object lens 17. The reflected laser beam is received by a focus error detection circuit 19 provided in the optical pickup head 15. Through the object lens 17, and a focusing error thereby is detected.
If the focusing error is within the focus servo range, a drive signal corresponding to the focusing error is applied to a focusing coil 21 by a focus servo control circuit 20 thereby to drive the focus servo.
If the focusing error is out of the focus servo range, the focus servo is temporarily switched off by a focus control circuit 24. Thereupon, the object lens, 17 is pulled back to a reference position (e.g., the lowermost position). By producing, in this state, a ramp voltage which gradually increases in its level and has, for example, a period of several seconds by the focus control circuit 24 and supplying this ramp wave to the focusing coil 21 as a drive signal, the object lens 17 is moved slowly in a direction perpendicular to the disc surface. At this time, the focusing state is watched to restore the focus servo at a point near focus.
In driving the focusing coil 21 by the ramp voltage, however, the speed of moving of the object lens 17 depends largely on states of friction of a shaft and a bearing supporting the object lens 17. Coefficients of friction of these portions vary depending upon factors such as roughness of the surfaces, accuracy in dimensions, physical properties of the material used of the shaft and bearing and environment in which they are placed. When, in particular, the object lens has stopped for a brief moment during its movement, a stick slip (i.e., a state in which the object lens moves irregularly, being repeatedly caught by the bearing) tends to occur due to difference between a dynamic coefficients of friction and a static coefficient of friction of the shaft and the bearing. For overcoming this problem, it has been attempted to employ a shaft coated with a coating containing fluorine as the shaft of the object lens. This approach has however proved costly and moreover inadequate to totally eliminate the problem.